For high performance semiconductor testers, sometimes referred to as automated test equipment or ATE, tester signals up to several gigahertz are funneled from test electronics through relatively large circuit boards known as device interface boards (DIB's) to the leads of one or more very compact devices-under-test or DUT's. The DUT's often are designed to include very high frequency analog signals to be included with high performance digital signals. These very high frequency analog signals include, for example, cellular telephone radio frequency signals, global positioning radio signals, wireless communication radio signals, etc. In today's high performance systems-on-a-chip (SOC), there are often several hundreds signal paths between the DUT and the tester electronics. In order to preserve fidelity for such high-frequency signals, the signal paths are constructed to provide a closely matched impedance (normally fifty ohms). Providing a closely matched impedance with a large number of signal paths is difficult.
The DIB is mounted on the automatic test equipment such that the electronic signals that are to be exchanged between the test electronics commonly referred to as a test head and the DUT are transferred through cables that connect to the DIB from the bottom side or tester side either directly with pogo pins or with the OSP, SMA, SMP or other high-speed connectors. The DUT is connected with a socket, which is mounted on the top side or DUT side of the DIB. The SMA coaxial connectors (Subminiature A originally designed by Bendix Scintilla Corporation), the OSP coaxial connectors (Omni-Spectra push-on originally designed by M/A Com of Lowell, Mass.), and the SMP coaxial connectors (originally invented by the Gilbert company) are commonly produced by companies such as Tyco Electronics Corporation, Berwyn, Pa.
The cables connected to a bottom side of the DIB and the DUT connected through a socket at the top side of the DIB are connected with conductive traces (strip lines or microstrip lines) and vias to connect traces of different layers. Further, the DIB contains various electronic components like Balanced/Unbalanced Transformers (BalUns), inductors, capacitors, and/or resistors, etc. to condition the test signals to ensure a good match between the tester resources and the DUT requirements. As the complexity of the systems-on-a-chip has increased the number of signals being transferred between the test head and the DUT, the thickness of the DIB has increased due to higher digital pin count.
As DIB's have increased in size, they require more thickness to realize the same stability. Additionally, multisite testing requirements (multiple DUT's are tested simultaneously) to reduce cost of test increase the wiring requirements of the DIB. This forces additional layers to be provided for routing all the wiring traces for all the signal channels. Currently the DIB requires up to twenty signal layers (approximately 5.08 mm) and in the future, at least thirty signal layers will be required, increasing the thickness by a factor of at least 1.5. As more layers are required the thickness of the DIB increases. As the numbers of signals is increasing, more of the connectors such as the pogo pins are needed. This creates more mechanical pressure on DIB, thus requiring a thicker printed circuit board so that the increase pressure can be sustained without damage to the printed circuit board.
FIG. 1 is a simplified block diagram of an automatic test equipment 100. The automatic test equipment 100 includes a tester mainframe 102 that is in communication with a test head 108. The test head 108 is connected to an DIB 106. Signals from the test head 108 may be routed to the DIB 106 through cable assemblies. In operation, the DIB 106 is electrically connected to a device under test (DUT) 104 for testing the DUT 104. For example, the automated test equipment (ATE) system 100 is for testing integrated circuits, and the DUT 104 may be a semiconductor device including integrated circuits that perform digital and analog functions. Examples of high performance analog functions are cellular telephone transmitters and receivers, digital wireless transmitters and receivers, radio frequency identification transmitters and receivers. Thus, signals from the test head 108 are routed through the DIB 106 to the DUT.
The tester mainframe 102 includes circuitry for generating test stimulus signals that are transferred through the test head 108 and the DIB 106 to the DUT 104 and evaluating test response signals received from the DUT 104 through the DIB 106 and the test head 108. The DUT 104 may be a packaged silicon die including an integrated circuit to be tested. The DIB 106 may also be connected to a probe interface card with the DUT 104 being a semiconductor wafer including integrated circuits to be tested that is mounted to the probe interface card.
FIG. 2 is a diagram of a cross-sectional view of a DIB printed circuit board 200. DIB printed circuit board 200 has a DUT socket 205 that is connected by compression connectors such as spring or pogo pin contacts 207 and 209 to metal wiring traces on the surface of the DIB printed circuit board 200. In the case where the DUT is an integrated circuit that performs digital and analog functions, the digital signals 230 are often segregated from the analog signals 235, especially for very high Radio Frequency (RF) signals. The analog signals 235 are transferred from the DUT through the pin 209 of the socket to a via 215. The via 215 is a hole drilled and plated in the DIB printed circuit board 200. The via 215 is connected to a metal wiring trace 220 that is connected to the coaxial connector 210. The coaxial connector 210 may be OSP, SMA, SMP or other high-speed connectors. The analog signals 235 are transferred between the coaxial connector 210 and the test head 108 of FIG. 1. The electronic components 226 are placed in contact with the wiring traces 220 to provide compensation and termination circuitry necessary to insure the signal fidelity of the analog signals 235.
As noted above, the increased circuit complexity and the growth in wiring density has increased the thickness of the DIB printed circuit board. As the DIB printed circuit board has increased in thickness, the performance has typically dropped for high-speed signals. One of the major detriments to the performance is the via 215. It is known in the art that vias have poor high-speed performance, since they represent an impedance discontinuity for GHz frequency analog signals 235. A long via 215 causes a problem for certain matching techniques, since components/circuitry 226 is too far away from DUT socket 205. It is difficult to place matching electronic components 225 in a more desirable location on the top side of the DIB printed circuit board 200, between the DUT socket 205 and the DIB printed circuit board 200, because there is insufficient space under the DUT socket 205 due to the increased RF pin count of the DUT.
Refer now to FIG. 3A for an example of a DIB printed circuit board 201 used for measuring the performance of very high frequency analog signals 235. In this example, the DIB printed circuit board 201 has an SMA coaxial connector 210 connected to the via 245. The via 245 is connected to the metal wiring trace 220 that is connected for this evaluation to a 50 Ω resistor 240 that is connected to a ground reference point 222. The SMA coaxial connector 210 has a shield that is placed in the vias 250. The vias 250 are also connected internally within the DIB printed circuit board 201 to the ground reference point.
In this example, the DIB printed circuit board 201 is constructed to be approximately 5.08 mm thick. This representative of the thickness employed for DIB printed circuit board 201 at the current time. As the complexity of integrated circuits increases, this thickness will be greater. The SMA connector is connected with the 50 Ω terminating resistor 240 after the via 245 that is approximately 5.08 mm (0.2″) in length.
FIG. 3B is a plot of the reflection of a signal passed through the SMA coaxial connector 210 through the via 245 to the 50 Ω terminating resistor 240, for the example of FIG. 3A. In the example of FIG. 3A, there is a −10 dB signal reflection 260 at 2 GHz. With the 5.08 mm length via 245, ten percent of the delivered power is reflected 260. The reflection 260 of this magnitude results in a loss of power and causes more difficulty in matching and compensation of the analog signals 235 to maintain signal fidelity.
Therefore, what is needed is a DIB printed circuit board constructed to provide a low loss in the wiring path of very high speed analog signals to maintain high signal quality and fidelity.